Non-explosive electrically initiated heat-ignitable actuator

ABSTRACT

A non-explosive, electrically initiated, heat-ignitable actuator constructed and arranged in the form of a biased, collapsible dual piston assembly. When the actuator is in the unactuated condition, the dual piston assembly is prevented from collapsing by an electrically triggerable dual shear pin and associated heat-ignitable wire arrangement. The shear pins and their associated wires are composed of a primarily alluminum and palladium bimetallic composition which when ignited produces a violent but non-explosive reaction which rapidly propagates to all portions of the material. The construction of the dual piston assembly is such that ignition of either or both of the shear pins causes the piston assembly to collapse and thereby bring about actuation of the actuator.

United States Patent [151 3,695, 16

Baur [451 Oct. 3, 1972 [5 NON-EXPLOSIVE ELECTRICALLY Primary Examiner-Milton Kaufman INITIATED HEAT-IGNITABLE Attomey-Frederick M. Arbuckle ACTUATOR [7 21 Inventor: Robert Baur, Los Angeles, Calif. [57] ABSTRACT A non-explosive, electrically initiated, heat-ignitable 7 .R 3] Asslgnee The Bunker Corporation actuator constructed and arranged in the form of a Oak Brook, lll. biased, collapsible dual piston assembly. When the ac- Flledi' p 1970 tuator is in the unactuated condition, the dual piston [21] Appl. No.: 76,952 assembly is prevented from collapsing by an electrically tnggerable dual shear pm and associated heat-1gnitable wire arrangement. The shear pins and their as- [52] US. Cl ..74/2 sedated wires are composed f a primarily anuminum 2; and palladium bimetallic composition which when ig- 1 o l nited produces a violent but non-explosive reaction 5 References Cited which rapidly propagates to all portions of the material. The construction of the dual piston assembly is such that ignition of either or both of the shear pins causes the piston assembly to collapse and thereby bring about actuation of the actuator.

24 Claims, 8 Drawing Figures PATENTED N 3 SHEET 2 [IF 3 A 7702M: y

NON-EXPLOSIVE ELECTRICALLY INITIATED HEAT-IGNITABLE ACTUATOR BACKGROUND OF THE INVENTION The present invention relates to fast acting, electrically initiatable actuators capable of providing relatively large mechanical forces. Such an actuator may be desired, for example, in order to provide for physically moving a mechanical member in response to an electrical initiating signal so as to initiate the automatic actuation of a remotely located mechanical device.

Known electrically initiatable actuators are of various types. Certain types involve the use of magnetic solenoid or servomechanism devices which have the disadvantages of being expensive, bulky, slow-acting and/or requiring excessive power. Other known types of actuators are of an explosive nature involving the use of explosive squibs, bridge-wire initiators, frangible diaphragms and similar devices of an explosive nature. While such explosive actuators may be made smaller and at less expense, they are more dangerous to use, handle, and test.

More recently developed types of electrically initiatable actuator devices have attempted to avoid the use of explosive actuation while retaining its advantages. Typical ones of such devices are described, for example, in U.S. Pat. Nos. 3,163,732 and 3,359,804. In a typical device of this nature a spring is maintained in a loaded condition by an electrically responsive keeper wire in axial tension. The construction and arrangement of the keeper wire is caused to be such that, when an electrical current of sufficient magnitude is caused to flow in the wire, the resulting heating causes the axial tensile strength of the wire to decrease below that required to retain the spring. The weakened keeper wire thus breaks and releases the spring to actuate any suitable device. While devices of this type are able to avoid the use of explosive actuation, they have been found to be of marginal reliability, relatively slow-acting, and/or relatively complex.

SUMMARY OF THE INVENTION In an exemplary embodiment of an improved actuator in accordance with the invention, the above referred to disadvantages of prior art actuators are to a very great extent overcome by the provision of a nonexplosive electrically initiated heat-ignitable actuator provided in the form of a biased, collapsible dual piston assembly. In the unactuated condition, the two pistons making up the dual piston assembly are prevented from collapsing by a pair of spaced heat-ignitable shear pins cooperating with respective ones of the pistons. Each shear pin is triggered by applying an appropriate initiating current to a respective heat-ignitable wire associated therewith. The heat-ignitable shear pins and their associated heat-ignitable wires are made from a bimetallic heat-ignitable composition comprised primarily of palladium and aluminum which when brought to ignition temperature produces a violent, but non-explosive, rapidly propagating reaction which results in a disintegration of all parts of the material. The collapsible dual piston assembly has a construction such that ignition of either or both of the shear pins will cause the assembly to collapse and thereby bring about actuation of the actuator.

The specific nature of the invention as well as the objects, advantages and features thereof will become apparent from the following description of a preferred embodiment thereof taken in conjunction with the accompanying drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an axial cross-sectional view of a typical embodiment of a non-explosive electrically initiatable actuator in accordance with the invention, the actuator being illustrated in its unactuated condition.

FIG. 2 is an axial cross-sectional view taken along the line 2-2 in FIG. 1.

FIG. 3 is an exploded perspective view of the actuator illustrated in FIGS. 1 and 2.

FIGS. 4 6 are axial cross-sectional views illustrating relative positions of the actuator elements prior to and after actuation.

FIGS. 7 and 8 are views illustrating how the actuator of FIGS. 1 3 may typically be employed with additional structure to initiate a desired remote mechanical movement.

Referring to the drawings in more detail and particularly to FIGS. 1 3, it will be seen that the actuator basically comprises a generally cylindrical housing 10 containing a collapsible dual piston assembly including outer and inner pistons 12 and 15 and a body portion 14, the assembly being prevented from collapsing by shear pins 20 and 30. The outer piston 12 is disposed within the left side of the housing 10 (as veiwed in FIGS. 1 3) and has a reduced cylindrical portion 12a extending from a hole 10a at the left end of the housing 10 (as viewed in FIGS. 1 3) and to which a biasing force F is applied. The inner piston 15 is disposed between the outer piston 12 and the body portion 14 and is slidably moveable within respective aligned axial bores 12b and 14a thereof. The heat-ignitable shear pins 20 and 30 are disposed in respective holes 15a and 15b of the inner piston 15 and respective holes 12c and 14b of the outer piston 12 and the body portion 14 whereby to maintain the illustrated uncollapsed condition of the piston assembly against the biasing force F. A threaded sleeve 17 is threaded into internal threads provided at the right end of the housing 10 and serves to maintain the uncollapsed dual piston assembly in a fixed position in the housing 10.'The right end of the housing 10 is sealed by potting material ,18 and the left end by an O-ring 41.

In order to provide for electrical initiation of the actuator illustrated in FIGS. 1 3, each of the heat-ignitable shear pins 20 and 30 has a respective fusible wire 22 or 32 associated therewith which passes therethrough and extends along the body portion 14 in recesses 14c (as best shown in FIG. 3) for soldering to respective ones of lead-in wires 26 and 36 to which initiating electrical current is applied. As shown, the lead-in wires 26 and 36 are fed into the housing 10 via the threaded sleeve 17 within which they are potted and are further supported in the housing 10 by respective stress-relieving ferrules 34 disposed in recesses 14d of the body portion 14. The housing 10, pistons 12 and 15, the body portion 14 and the threaded sleeve 17 are all of insulative material in order to prevent shorting of the shear pins 20 and 30 and their associated heat-ignitable wires 22 and 32.

It will be understood from FIGS. 1 3 that the actuator may typically be assembled by first forming a subassembly comprised of the pistons 12 and 15, the body portion 14, and the heat-ignitable shear pins 20 and 30 along with their associated heat-ignitable wires 22 and 32 appropriately soldered to the lead-in wires 26 and 36 carried by ferrules 34. This subassembly is then inserted into the left end of housing behind the O-ring 41 with the reduced cylindrical portion 12a extending from the hole 10a at the right end of the housing 10 and with the lead-in wires 26 and 36 extending from the right side of the housing 10. The threaded sleeve is then threaded into the right end of the housing 10 and the potting material 18 then applied. The O-ring 41 together with the potting material 18 serves to provide satisfactory sealing for the elements of the subassembly within the housing 10.

Preferably, the heat-ignitable shear pins 20 and 30 and their associated heat-ignitable wires 22 and 32 are made from a bimetallic composition which when brought to operating or ignition temperature will alloy violently and exothermically, resulting in deflagration without the support of oxygen. Ignition is by heat alone, requiring only the exposure of the composition to the operating temperatures. When this reaction is triggered, temperatures in excess of the boiling point of the constituents are reached and, once started, the reaction will continue until the alloying of all available materials to which the reaction is able to propagate is complete, or unless it is cooled by some mechanical means below the operating temperature. The reaction products are normally an alloy of the participating materials. It is important to note that, although the reaction is violent and very rapid (of the order of milliseconds), the reaction is not of an explosive nature and there is no shock or detonation. The only energy released is thermal (approximate minimum temperature 2,800 C. 325 calories per gram and 2,890 calories per cubic centimeter).

Suitable bimetallic composition which may be used for the heat-ignitable shear pins 20 and 30 and the heat-ignitable wires 22 and 32 are available from the Pyrofuze Corporation, an affiliate of Sigmund Cohn Corporation, 121 South Columbus Avenue, Mount Vernon, New York. The product is sold under the registered trademark PYROFUZE, is offered mainly as a composite of essentially palladium and aluminum, and may be purchased in a variety of structural forms. For the exemplary embodiment of the invention being described herein, the heat-ignitable shear pins 20 and 30 are typically formed from 0.001 inch thick PYROFUZE foil containing an outer shell of palladium and an inner core of aluminum, the aluminum constituting 99 percent of the composition. The heat-ignitable wires 22 and 32 are each typically provided as a composite braid formed by first braiding eight single strands of 0.003 inch diameter PYROFUZE I-II-K wire so as to form an inner braid, and then braiding an additional eight single strands of 0.004 inch diameter PYROFUZE I-Il-K wire over the inner braid. Each HI-K PYROFUZE wire used in the braiding has an inner core containing 0.30 percent silicon (maximum), 0.40 percent iron (maximum), 0.10 percent copper (maximum), .10 percent zinc (maximum), 0.05-0.20 percent manganese, 4.5-5.6 percent magnesium, 0.05-0.20 percent chromium, and the balance aluminum, and an outer shell containing 5 percent ruthenium and the balance palladium.

. The above-described foil used for each of the heatignitable shear pins 20 and 30 may typically be about 2.6 inches long and 0.3 inch wide and is rolled onto its respective heat-ignitable wire 22 or 32 to form a cylinder encircling its respective wire and having an overall diameter of about 0.07 inch for insertion into its respective holes 12c and 15a or 15a and 14b, as illustrated in FIGS. 1 3. In order to trigger high speed ignition starting from a known location, as well as to provide a desired input resistance for the actuator, it is preferred that one of the soldered connections between each heat-ignitable wire 22 or 32 and its respective lead-in wire 26 or 36 be made using only a single one of the eight strands of the inner braid, as illustrated by numerals 22a and 32a in FIGS. 1 and 2. Of course, care must be taken in soldering to the heat-ignitable wires to prevent ignition. This is no problem since heat-ignitable wires of the type described are able to withstand temperatures up to about 800 F. without igniting. The overall length of the heat-ignitable wires 22 and 32 may typically be about 1.1 inch and 1.7 inch, respectively, in a housing 10 which is typically of about 2 inches in length and about 0.5 inch outside diameter. For such an actuator having the typical heat-ignitable shear pins 20 and 30 and heat-ignitable wires 22 and 32, as described above, it has been found that an input current of about 7.5 amperes to each pair of lead-in wires 26 and 36 is sufficient to provide sufficient heating to reliably trigger the ignition of each of the heat-ignitable wires 22 and 32, starting at the single strand locations 22a and 33a. The reaction then rapidly propagates (in a manner somewhat similar to what occurs in a conventional fuse but much more rapidly) to their respective shear pins 20 and 30, each of which will then also be ignited as a result of the heating of the adjacent respective heat-ignitable wire passing therethrough. The resulting ignition and disintegration of the heat-ignitable shear pins 20 and 30 removes their restraining effect on the movement of the pistons 12 and 15 so as to thereby permit the dual piston assembly to collapse in response to the applied force F to complete actuation of the actuator, as will next be considered with reference to FIGS. 4 6.

FIG. 4 is essentially the same as FIG. 1 and is provided as a reference for FIGS. 5 and 6 which illustrate two possible resulting actuated or collapsed positions of the outer and inner pistons 12 and 15 following ignition of the actuator. It will be noted that, for greater clarity, neither the shear pins 20 and 30 not their associated heat-ignitable wires 22 and 32 are shown in FIGS. 5 and 6.

Considering FIG. 5 in more derail, it will be seen that this figure illustrates the resulting actuated or collapsed position of the pistons 12 and 15 for the situation where only the shear pin 30 is ignited, in which case rightward movement of the outer piston 12 by the distance D is obtained as a result of the right end of the inner piston 15 moving to the right into the bore 14a of the body 14. FIG. 6 on the other hand illustrates the situation where only the shear pin 20 is ignited, in which case the same desired rightward movement of the outer piston 12 by the distance D is obtained as before, but this time it occurs as a result of the outer piston 12 moving to the right so that its bore 12b receives a greater portion of the left end of the inner piston 15.

It should now be evident that, if both of the shear pins 20 and 30 are ignited, which is normally the case, the desired rightward movement of the outer piston D is again obtained, but in such case will usually be a result of a combination of the piston movements illustrated in FIGS. 5 and 6, since both types of movement will be able to occur. The resulting position of the inner piston will thus ordinarily be somewhere intermediate that shown in FIGS. 5 and 6, depending primarily on the relative times of ignition of the shear pins and 30. It will be understood that this resulting intermediate position of the piston 15 is not significant, since the important requirement is that the desired rightward movement D of the reduced cylindrical portion 12a of the outer piston 12 be achieved in response to the applied force F, and this requirement will be met as long as at least one of the shear pins 20 and is ignited. It has been found that with a bias force F of, for example, eighty pounds applied to an embodiment of an actuator of the type described herein, only approximately 40 milliseconds is required following the application of initiating current to the lead-in wires 26 and 36 for the actuator to reach the fully actuated condition where the reduced cylindrical portion 12a of the outer piston 12 has moved the desired distance D.

Having described with reference to FIGS. 1 6 the construction and operation of an exemplary embodiment of an actuator in accordance with the invention, reference is next directed to FIGS. 7 and 8 which illustrate how such an actuator 50 may typically be provided within a suitable housing 55 along with other structure for the purpose of initiating a desired mechanical operation. FIG. 7 illustrates the actuator in its unactuated condition with the applied force F (FIGS. 1 and 2) being provided by compressed springs 58 having cooperating members 59 urging a pair of pivotable latches 62 in respective clockwise and counter clockwise directions so that they act against the reduced cylindrical portion 12a of the actuator 50.

FIG. 8 illustrates the actuator 50 in its actuated condition with the reduced cylindrical portion 12a having moved within the housing 10, and with the latches 62 having been rotated, as indicated by the arrows A, to free the springs 59 to move in the direction indicated by the arrows B so as to thereby be able to act on any suitable structure which might be provided to obtain a resulting desired mechanical motion.

Although the invention has been described in connection with a particular exemplary embodiment, it is to be understood that the construction, arrangement, fabrication and/or use of the invention is subject to considerable variations and/or modifications without departingfrom the scope of the invention as defined in the appended claims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

l. A non-explosive, electrically initiated, heat-ignitable actuator comprising:

a housing,

a collapsible piston assembly disposed within said housing,

at least one heat-ignitable shear pin cooperating with said collapsible piston assembly to prevent collapse thereof, andinitiating means for igniting said shear pin in response to an applied electrical signal.

2. The invention in accordance with claim 1, wherein said shear pin is in the form of a hollow cylinder and said initiating means includes a heat-ignitable wire passing through said cylinder for causing ignition of said shear pin as a result of said wire being ignited in response to said electrical signal being applied thereto.

3. The invention in accordance with claim 1, wherein said shear pin and wire are of a heat-ignitable material which when brought to ignition temperature produces a violent and rapidly propagating but non-explosive reaction resulting in disintegration of the material.

4. The invention in accordance with claim 1, wherein said heat-ignitable material is a bimetallic composition comprised primarily of aluminum and palladium.

5. The invention in accordance with claim 1, wherein said shear pin is formed by rolling a foil of said heat-ignitable material onto said wire so as to form a cylinder encircling said wire.

6. The invention in accordance with claim 1, wherein said wire is provided with a substantially reduced diameter portion at a predetermined location thereof in order to establish an initial starting point for ignition of the wire in response to the applied electrical signal.

7. The invention in accordance with claim 1, wherein a second heat-ignitable shear pin is provided cooperating with said collapsible piston assembly so that ignition of either shear pin permits collapsing of said assembly.

8. The invention in accordance with claim 1, wherein said collapsible piston assembly includes first and second pistons whose movements within said housing are respectively prevented by the shear pins.

9. The invention in accordance with claim 1, wherein said shear pins are of a heat-ignitable material which when brought to ignition temperature produces a violent and rapidly propagating but non-explosive reaction resulting in disintegration of the material, and wherein said initiating means includes a heat-ignitable wire of said material cooperating with each shear pin for causing ignition thereof as a result of said wire being ignited in response to said electrical signal being applied thereto.

10. A non-explosive electrically initiated, heat-ignitable actuator comprising:

a housing,

a collapsible dual piston assembly disposed within said housing,

first and second heat-ignitable members cooperating with said dual piston assembly in a manner so as to prevent collapse thereof so long as neither member is ignited, and

initiating means for igniting said members in response to an applied electrical signal.

11. The invention in accordance with claim 10, wherein said collapsible dual piston assembly includes inner and outer pistons and a body portion, said outer piston being movable with respect to said housing and said inner piston being disposed between said outer piston and said body portion and being relatively movable with respect to both, one of said heat-ignitable members being disposed so as to prevent relative movement between said inner piston and said outer piston and the other of said heat-ignitable members being disposed so as to prevent relative movement between said inner piston and said body portion.

12. The invention in accordance with claim 1 1, wherein said members are each provided as a shear pin, and said initiating means includes first and second heatignitable wires cooperating with respective ones of said shear pins for causing ignition thereof as a result of its respective wire being ignited in response to said electrical signal being applied thereto.

13. The invention in accordance with claim 12, wherein each shear pin is provided in the form of a hollow cylinder through which its respective wire is passed.

14. The invention in accordance with claim 12, wherein said shear pins and wires are of a heat-ignitable material which when brought to ignition temperature produces a violent and rapidly propagating but non-explosive reaction resulting in disintegration of the material.

15. The invention in accordance with claim 11, wherein said outer piston is slidably movable within said housing and said inner piston has its end portions slidably movable in respective bores provided in said outer piston and said body portion, and wherein one of said members is disposed so as to prevent relative movement of said inner piston in the bore of said outer piston and the other of said members is disposed so as to prevent relative movement of said inner piston in the bore of said body portion.

16. The invention in accordance with claim 15, wherein said members are each provided as a shear pin, and said initiating means includes first and second heatignitable wires cooperating with respective ones of said shear pins for causing ignition of each as a result of its respective wire being ignited in response to said electric'al signal being applied thereto.

17. The invention in accordance with claim 16, wherein said shear pins and wires are of a heat-ignitable material which when brought to ignition temperature produces a violent and rapidly propagating but non-explosive reaction resulting in disintegration of the material.

18. The invention in accordance with claim 16, wherein said heat-ignitable material is a bimetallic composition comprising primarily aluminum and palladium.

19. The invention in accordance with claim 16, wherein said housing also includes means for maintaining the uncollapsed dual piston assembly in a fixed position in said housing.

20. The invention in accordance with claim 19, wherein said housing includes means adapted to receive an applied force acting so as to urge the respective ends of said inner piston more deeply into their respective bores against the restraint provided by their respective shear pins, and wherein the ends of said inner piston are each able to move a sufficient distance into their respective bores so as to cause the piston assembly to collapse by at least a predetermined minimum amount in response to an applied force if at least one of said shear pins is ignited.

21. The invention in accordance with claim 20,

wherein said initiating means includes lead-in wires fed into said housing and electrically connected to said heat-ignitable wires so as to permit applying an initiating current thereto sufficient to cause ignition thereof.

22. The invention in accordance with claim 21, wherein each heat-ignitable wire has a greatly reduced diameter at a predetermined location for establishing a starting point for ignition thereof in response to an applied initiating current.

23. The invention in accordance with claim 21, wherein said housing also includes means for sealing said piston assembly therein.

24. The invention in accordance with claim 16, wherein means are additionally provided responsive to the collapse of said piston assembly for initiating the performance of a mechanical operation. 

2. The invention in accordance with claim 1, wherein said shear pin is in the form of a hollow cylinder and said initiating means includes a heat-ignitable wire passing through said cylinder for causing ignition of said shear pin as a result of said wire being ignited in response to said electrical signal being applied thereto.
 3. The invention in accordance with claim 1, wherein said shear pin and wire are of a heat-ignitable material which when brought to ignition temperature produces a violent and rapidly propagating but non-explosive reaction resulting in disintegration of the material.
 4. The invention in accordance with claim 1, wherein said heat-ignitable material is a bimetallic composition comprised primarily of aluminum and palladium.
 5. The invention in accordance with claim 1, wherein said shear pin is formed by rolling a foil of said heat-ignitable material onto said wire so as to form a cylinder encircling said wire.
 6. The invention in accordance with claim 1, wherein said wire is provided with a substantially reduced diameter portion at a predetermined location thereof in order to establish an initial starting point for ignition of the wire in response to the applied electrical signal.
 7. The invention in accordance with claim 1, wherein a second heat-ignitable shear pin is provided cooperating with said collapsible piston assembly so that ignition of either shear pin permits collapsing of said assembly.
 8. The invention in acCordance with claim 1, wherein said collapsible piston assembly includes first and second pistons whose movements within said housing are respectively prevented by the shear pins.
 9. The invention in accordance with claim 1, wherein said shear pins are of a heat-ignitable material which when brought to ignition temperature produces a violent and rapidly propagating but non-explosive reaction resulting in disintegration of the material, and wherein said initiating means includes a heat-ignitable wire of said material cooperating with each shear pin for causing ignition thereof as a result of said wire being ignited in response to said electrical signal being applied thereto.
 10. A non-explosive electrically initiated, heat-ignitable actuator comprising: a housing, a collapsible dual piston assembly disposed within said housing, first and second heat-ignitable members cooperating with said dual piston assembly in a manner so as to prevent collapse thereof so long as neither member is ignited, and initiating means for igniting said members in response to an applied electrical signal.
 11. The invention in accordance with claim 10, wherein said collapsible dual piston assembly includes inner and outer pistons and a body portion, said outer piston being movable with respect to said housing and said inner piston being disposed between said outer piston and said body portion and being relatively movable with respect to both, one of said heat-ignitable members being disposed so as to prevent relative movement between said inner piston and said outer piston and the other of said heat-ignitable members being disposed so as to prevent relative movement between said inner piston and said body portion.
 12. The invention in accordance with claim 11, wherein said members are each provided as a shear pin, and said initiating means includes first and second heat-ignitable wires cooperating with respective ones of said shear pins for causing ignition thereof as a result of its respective wire being ignited in response to said electrical signal being applied thereto.
 13. The invention in accordance with claim 12, wherein each shear pin is provided in the form of a hollow cylinder through which its respective wire is passed.
 14. The invention in accordance with claim 12, wherein said shear pins and wires are of a heat-ignitable material which when brought to ignition temperature produces a violent and rapidly propagating but non-explosive reaction resulting in disintegration of the material.
 15. The invention in accordance with claim 11, wherein said outer piston is slidably movable within said housing and said inner piston has its end portions slidably movable in respective bores provided in said outer piston and said body portion, and wherein one of said members is disposed so as to prevent relative movement of said inner piston in the bore of said outer piston and the other of said members is disposed so as to prevent relative movement of said inner piston in the bore of said body portion.
 16. The invention in accordance with claim 15, wherein said members are each provided as a shear pin, and said initiating means includes first and second heat-ignitable wires cooperating with respective ones of said shear pins for causing ignition of each as a result of its respective wire being ignited in response to said electrical signal being applied thereto.
 17. The invention in accordance with claim 16, wherein said shear pins and wires are of a heat-ignitable material which when brought to ignition temperature produces a violent and rapidly propagating but non-explosive reaction resulting in disintegration of the material.
 18. The invention in accordance with claim 16, wherein said heat-ignitable material is a bimetallic composition comprising primarily aluminum and palladium.
 19. The invention in accordance with claim 16, wherein said housing also includes means for maintaining the uncollapsed dual piston assembly in a fixed position in said housiNg.
 20. The invention in accordance with claim 19, wherein said housing includes means adapted to receive an applied force acting so as to urge the respective ends of said inner piston more deeply into their respective bores against the restraint provided by their respective shear pins, and wherein the ends of said inner piston are each able to move a sufficient distance into their respective bores so as to cause the piston assembly to collapse by at least a predetermined minimum amount in response to an applied force if at least one of said shear pins is ignited.
 21. The invention in accordance with claim 20, wherein said initiating means includes lead-in wires fed into said housing and electrically connected to said heat-ignitable wires so as to permit applying an initiating current thereto sufficient to cause ignition thereof.
 22. The invention in accordance with claim 21, wherein each heat-ignitable wire has a greatly reduced diameter at a predetermined location for establishing a starting point for ignition thereof in response to an applied initiating current.
 23. The invention in accordance with claim 21, wherein said housing also includes means for sealing said piston assembly therein.
 24. The invention in accordance with claim 16, wherein means are additionally provided responsive to the collapse of said piston assembly for initiating the performance of a mechanical operation. 